1. Field of the Invention
The present invention relates to a cleaning solution and, in particular, to a cleaning solution and a cleaning method for removing an organic EL material adhering to a mask in a vacuum vapor deposition step in the production of a low molecular weight organic EL device.
2. Description of Related Art
Flat panel displays are attracting attention as display devices, and among them display devices equipped with a liquid crystal display device or an organic EL device are excellent. The liquid crystal display device has low power consumption but requires external lighting (back light) in order to obtain a bright screen, whereas the display device equipped with an organic EL device has the characteristics of not requiring a liquid crystal display device-type back light since the organic EL device is a self-emitting device, thus saving power, and also has characteristics such as high luminance and a wide viewing angle.
With regard to the organic EL device, there are two types, depending on the type of organic material, that is, a low molecular weight organic EL device and a polymer organic EL device, the two types employing different device production processes. The former employs film formation by a vapor deposition method, and the latter employs film formation by a spin coating method or an inkjet method after dissolution in a solvent.
With regard to the low molecular weight organic EL device, a layer-form structure is formed by vacuum vapor deposition using a mask, the layer-form structure comprising in turn on a glass substrate, for example, (1) an anode, (2) a hole-injecting layer, (3) a hole-transporting layer, (4) a light-emitting layer, (5) an electron-transporting layer, and (6) a cathode.
The mask generally used is a metal mask produced by etching, etc. of a metal such as SUS with a thickness of on the order of 0.1 mm, but as a mask that enables processing with higher precision, a mask produced by anisotropic etching of single crystal silicon having (100) or (110) orientation has been proposed (JP, A, 2002-110345, JP, A, 2002-305079, and JP, A, 2002-313564).
As one example of the structure of the low molecular weight organic EL device, there has been disclosed a multilayer structure comprising, for example, (1) indium tin oxide (ITO) as the anode, (2) a single layer of copper (II) phthalocyanine (CuPc) as the hole-injecting layer, (3) a single layer of N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine (NPB) as the hole-transporting layer, (4) a layer of tris(8-quinolinolato) aluminum (Alq3) with 2% coumarin-6 added thereto as the light-emitting layer, (5) a single layer of Alq3 as the electron-transporting layer, and (6) a layer of an Mg/In alloy as the cathode (JP, A, 2003-109757).
In this example, CuPc is used as the hole-injecting layer, but a hole-injecting layer might not be provided in some cases. NPB is usually used as the hole-transporting layer.
The light-emitting layer is obtained by using a chelate metal complex or a fused polycyclic aromatic compound as a host and doping with various types of dopant. For blue light emission the fused polycyclic aromatic compound 2-tert-butyl-9,10-di(naphthalen-2-yl)anthracene (TBADN), etc. is used, and for red or green light emission the chelate metal complexes Alq3 and bis(benzoquinolinato) beryllium complex (BeBq2), etc. are used.
When TBADN is used as the light-emitting layer, an electron-transporting layer (e.g., Alq3) is generally used, and when the light-emitting layer is a chelate metal complex such as Alq3, the electron-transporting layer can sometimes be omitted (JP, A, 2003-257664).
For pattern formation of these layers, it is necessary to bring the mask close to the substrate and carry out vacuum vapor deposition, via the mask, of the cathode, the hole-injecting layer, the hole-transporting layer, the light-emitting layer, the electron-transporting layer, and the anode; in particular, it is difficult to produce a vapor deposition mask for fine patterning of an RGB layer because it is a high definition mask, and furthermore it is very expensive. However, in pattern formation of organic layers in the low molecular weight organic EL device, if vapor deposition is carried out several times using the same mask, since organic materials are deposited on and adhere to the mask, the high definition pattern of the mask cannot be transferred to the substrate accurately. Therefore, in order to realize a high definition mask pattern, once an expensive mask has been used a few times it has to be disposed of, and this makes mass production difficult from the viewpoint of production cost. In the organic EL field, which is in the development stage, reducing the cost by reusing the mask has not so far been attempted or investigated.